Precise orchestration of myriad of event is necessary for normal skeletal development and maintenance of its integrity. This proposal focuses on endochondral bone formation and the roles parathyroid hormone (PTH), PTH- related peptide (PTHrP) and PTH/PTHrP receptors play in the complex interrelationships between the PTH/PTHrP system and skeletal growth factors and their receptors, and the several collagens and non- collagenous proteins that are integral to bone biology. PTHrP is essential for normal skeletal maturation, as evidenced by striking abnormalities in mice whose PTHrP gene have been largely deleted by homologous recombination (PTHrP-less). Recently, receptors that bind both PTH and PTHrP have been cloned from several species; they have the unique property of binding both a circulating hormone important to calcium homeostasis, and a presumed paracrine/autocrine peptide with obvious importance to bone biology. These receptors also activate multiple effector molecules, including adenylate cyclase and phospholipase C. Numerous growth factors have also been shown to impact critically on skeletal development and homeostasis; among these, current evidence suggests that members of the insulin-like growth factors (IGFs), transforming growth factor beta (TGFbeta), and bone morphogenetic proteins (BMPs) either regulate, or are regulated by the PTH/PTHrP system. We plan to use a variety of approaches to understand the mechanism by which PTH and PTHrP modulate skeletal expression of genes for these growth factors, and to define influences these growth factors have on PTHrP and PTH/PRHrP receptor mRNA and protein expression. These interrelationships will be explored by both morphological criteria and molecular biological approaches in several systems. Aim I details our initial descriptive analyses to characterize gene expression in late fetal skeletal development in normal rats, rats up to 46 weeks of age, both normal and PTHrP-less fetal mice, and in chondrocytic and osteoblastic cells in primary culture. We have developed systems which we can perturb to analyze these relationships at increasingly complex levels of organization--from primary cells in culture, to organ explants and intact integrating in vitro and in vivo studies, we expect to define mechanisms at the cellular and organ level and relate the relevance of these findings to the whole animal. Aim II mainly analyzes the effects of PTH (1-84) and PTHrP (1-141) on expression of genes for growth factors, several collagens and non-collagenous proteins. Aim III examines the reciprocal relationship; that is, the influences of these growth factors on expression of PTHrP and PTH/PTHrP receptor mRNA and protein. Greater understanding of skeletal homeostasis will, in turn, contribute to bone biology in general and lead to more rational therapy for diseases like osteoporosis.